This invention is directed to drive circuits for induction motors used in air conditioning systems, and more particularly to a drive circuit for an induction motor used in an air conditioning system which is adapted for installation in a railway vehicle.
In recent years it has become almost universal practice to provide air conditioning systems in all types of passenger railway vehicles. In the case of rapid transit and other electrically driven railway vehicles, the only practical power source for the compressor motors and fan motors of such air conditioning systems in the high-voltage DC source used to power the vehicle, which may be provided by means of a pickup or shoe contacting a third rail or by means of a pantograph contacting an overhead wire. This power source has not been entirely satisfactory for direct application to DC motors because of the severe transients which are often developed as the vehicle passes over switches and open blocks where power may be temporarily interrupted.
Moreover, due to track position requirements, power is drawn from alternate shoes located on either side of the vehicle. While these shoes are electrically connected in parallel, establishing and breaking electrical contact is not always done smoothly or in complete synchronization. Therefore, the vehicle can be completely without power many times during normal operation. This sudden interruption and reapplication of the power souce establishes electrical transients and other disturbances.
Furthermore, the power supply voltage, which typically measures in excess of 600 volts and may vary widely over different sections of track, is difficult to regulate and presents special motor insulation and stability problems. Attempts at reducing the voltage applied to the DC motors by provision of series resistors between the power source and the motors have not only undesirably wasted power, but have also created additional cooling and ventilation problems for vehicle manufacturers.
Another problem with prior art DC motors is that they require periodic brush replacement and frequent overhaul of commutator segments due to arcing across the segments. The high maintenance required of these motors makes their use in railway vehicles unnecessarily costly and detracts from the reliability of the vehicle.
One potentially successful solution to this problem has been to use induction motors in the air conditioning system, in conjunction with an onboard DC to AC converter to convert the third-rail direct current to alternating current. Induction motors, which have no commutators and brushes, have proven to be substantially more reliable than brush motors. The use of induction motors for vehicle ventilation with their inherent reliability substantially reduces the possibility of car ventilation loss. This is particularly important because a paradoxial consequence of complete air conditioning in railway vehicles is the total dependence on forced ventilation of the occupied space. The limited cooling capacity of the system necessitates controlled circulation of ventilating air which places a great reliability burden on vehicle air moving equipment, in particular the ventilating blowers. Any failure of the ventilating equipment renders an occupied vehicle uninhabitable in a very short time due to increased temperature and buildup of moisture, resulting in extreme discomfort to the passengers.
Previous approaches to providing onboard power converters for induction motors have not been entirely satisfactory. For instance, one approach has been to provide a motor-alternator (MA) set on the vehicle to convert the third-rail direct current to alternating current. Difficulties have been encountered due to the weight and complexity of MA sets, which add unnecessarily to the vehicle gross weight and hence increase cost of manufacture and operation of the vehicle. Furthermore, MA sets themselves require periodic brush replacement and maintenance and therefore at best achieve only a partial reduction in maintenance. Prior art attempts at providing static solid-state motor drive circuits for this purpose have also been unsuccessful because the adverse electrical and physical environment in which such motor drive circuits must operate has heretofore prevented the degree of stability and reliability necessary for successful application to a railway vehicle air conditioning system. In addition, the repeated interruptions of third rail power and electrical transients present in a third rail power source, which were previously described, presents an additional operating condition, which in the past, has been particularly troublesome to solid state motor drive circuits operating on railway vehicles.
Other car power sources may also be encountered. For example if train operation from single phase or three phase alternating current supplied from a catenary construction is required, transformer-rectifier combinations are normally used to provide the high voltage DC supply for traction and other auxiliary functions such as air conditioning.
An object of the present invention is the provision of a solid-state circuit for driving an induction motor in an air conditioning system of a railway vehicle from the high voltage DC supply which powers the vehicle.
It is also an object of this invention to provide a solid-state circuit for converting direct current to alternating current of varying frequency utilizing feedback means to provide satisfactory starting and operation of a number of induction motors; operating over wide load ranges. The individual motors can be of substantially different horsepower ranges, and can be operated singly or in any combination.
An additional object of the invention is a novel semi-conductor direct current chopper utilizing a novel commutation scheme.
A further object of the invention is to provide a solid-state circuit which is free from many of the transient induced malfunctions normally found in this class of equipment.